Intersymbol interference (ISI) is a serious problem in digital communications systems. Various ways have been developed to reduce ISI at both the transmitter end and the receiver end of a communications system. Existing methods attempt to equalize transmitted data signals, with the objectives of correcting for the effects of channel attenuation and complete cancellation of ISI. A conventional serial receiver consists of an analog front end that generally includes a continuous time linear equalizer (CTLE), a sampler that quantizes the analog input into digital values, a decision feedback equalizer (DFE) that uses the quantized data to adaptively feedback a correction signal to the input of the receiver, and a timing recovery unit.
In many communication channels, ISI occurs at frequencies that are significantly lower than the data rate. The presence of ISI at low frequencies can be attributed to skin effect loss in communication channels that connect the transmitter to the receiver. The skin effect causes channel attenuation to be more proportional to the square root of frequency rather than being linear with frequency. Conventional components in serial links such as a transmitter equalizer, CTLE, and DFE typically only compensate for ISI at frequencies down to around one-fifth the data rate, and therefore leave significant amounts of residual ISI.